In 1967 the Tandem Scanning Reflected Light Microscope was introduced by Drs. Petran and Hadravsky in Czechoslovakia. The need to view internal cellular structures with a high degree of resolution and contrast led to the development of this new theory of microscope structure which is delineated in U.S. Pat. No. 3,517,980 issued on June 30, 1970. In the disclosure of that seminal patent, the inventors describe, among other things, elimination of undesirable Airy disk imaging by the use of a rotating Nipkow disk for simultaneous scanning of the object and the image field. The Nipkow disk incorporated a multitude of minute holes extending through the disk thickness and arranged in Archimedean spirals, each corresponding to the zero maximum Airy disk for an imaged point.
One objective in ideal tandem scanning techniques is to obtain a uniform light intensity over the entire field of view. Accurately maintaining the level and uniformity of light is critical to achieve a desired measurement accuracy which often exceeds the norma theoretical resolution of conventional microscopes. Since tandem scanning microscopy devices and techniques are especially useful for viewing translucent materials exhibiting low contrast, uniform light intensity is critical, as for example in biological systems where the need for precision requires no elaboration. Likewise, in integrated circuit manufacture field-of-view light intensity uniformity is important, especially where computer programs are used to automatically identify or measure details for automatically controlling circuit production processes. Thus, high clarity images facilitate uniform manufacture of features smaller than one micrometer in size.
Moving now to a general explanation of Nipkow disks, the relative area of the pinholes to the disk surface area comprises only a small percentage. For example, a 1% disk means that 1% of the surface area is defined by holes. Consequently, such a disk can pass only 1% of impinging random light. In the case of tandem scanning microscopes, only 1% of the random light returned from out-of-focus planes within the object being observed passes through the disk to the eyepiece. Thus, where non-random light is focused through a particular pinhole, in comparison to the passage of a mere 1% of random light a significant contrast ratio to non-random light--the focused images observed with a tandem scanning microscope--is established. It should also be evident that, in general, a lower aperture area (1%) disk provides a higher contrast ratio and, therefore, is more desirable than a higher area (2%) disk. However, this is true only when the holes are evenly distributed over the entire active disk area. A 1% disk can be made by covering all of the holes in a 2% disk between an angle of 90 degrees and 180 degrees and between 270 degrees and 360 degrees. Since such a disk is made by eliminating half of the holes on a 2% disk, it is properly a 1% disk but the contrast ratio of images produced with such a disk will be no better than that with the original 2% disk. Accordingly, better contrast ratio is obtained when local concentrations or small clumps of holes are avoided and the holes are distributed over the entire active disk annulus.
One problem commonly associated with the Archimedean spiral hole pattern described above, is streaking. Streaking occurs when a tandem scanning microscope or a video camera with a frame grabber to grab a single frame for use in a computer, are employed to produce static photographic images of the target. More particularly, streaking results due to mismatching of the exposure time or the frame speed to the disk rotation speed. For example, since each scan of the target provides, by itself, substantially complete coverage of the entire field of view, the exposure time may be equivalent to the time required for the disk to rotate through eight and one-half scans, i.e. eight and one-half scanned views. A complete scan may be a single spiral or a cycle of radially staggered spirals. It is important that Archimedean spirals or cycles of spirals have beginning and end points. So, while eight scans will provide a high quality picture, the extra half-scan will result in picture streaking because the scan is stopped between the spiral or scan end points. The image produced by eight successive scans will be intense enough that one extra scan line from one hole will not greatly interfere with the picture quality. However, where several extra scan lines are grouped together, as caused by an unbalanced intensity of one target portion scan to the exclusion of the remainder, i.e. extra one half scan, a significant deterioration of picture quality results. For this reason, since the introduction of the Nipkow disk based Tandem Scanning Microscope, variations of the basic invention have been developed.
For example, McCarthy et al. in U.S. Pat. No. 4,802,748, describe a Nipkow disk for tandem scanning microscopy attempting to maximize light uniformity by relying on equidistant linear hole spacing along the Archimedean spiral; i.e. general uniform distribution on the disk. The patent disclosure stresses the importance of uniform linear spacing along the spiral and also describes the criticality to the operation of the confocal tandem scanning light microscope that 1) holes on opposite sides of the disk be exact conjugates, 2) the illumination of the image be evenly distributed, and 3) pinhole size and placement be such that overlapping scan lines are minimized. However, McCarthy, as the others in the art, does not recognize or appreciate possible benefits to having the pinholes uniformly distributed within any given segment of the disk.